That confused me a bit too, but what I suspect is that owning a pet is kind of like having a permanent baby, but without many of the real challenges a human child presents.

Your dog can be cute and child-like and playful for its entire life, but is also far more self-sufficient than a human child is in the first year or two of its life.

It’s kind of like you get to be a make-believe parent without any of the difficult parts.

>That confused me a bit too, but what I suspect is that owning a pet is kind of like having a permanent baby,

No, he's saying that having a puppy is something a child does, something that's normal for children. If you're doing things that only children should be doing, you are infantilizing yourself (changing your brain in ways that prevents you from growing up properly).